Nitrogen recombination dynamics at Cu(111): Rotational energy release and product angular distributions

Nitrogen atoms adsorbed on Cu(lll) desorb thermally from an ordered Cu(100) - c(2 x 2)N phase in a sharp, zero order desorption feature near 700 K with an activation barrier of 143 kJ mol(-1). Detailed N-2 product rovibrational state distributions have been measured following recombinative desorption from a 700 K Cu(lll) surface exposed to a N atom beam, with an equilibrium N coverage theta(N)less than or equal to 10(-2) ML. Although desorbing N-2 is translationally and vibrationally hot, with a vibrational temperature of 5100 K and 4.2 eV of translational excitation perpendicular to the surface, rotation is excited with a temperature of just 910(+/- 50) K for the vibrational ground state and 840(+/-250) K for (upsilon = 1). The energy released during recombinative desorption channels effectively into translational and vibrational motion, but not into rotational excitation. The angular distribution of recombinatively desorbed N-2 is sharply peaked along the surface normal, P(theta) = cos((28+/-1)) theta, indicating a mean energy release of 0.28 eV into translation parallel to the surface. This is inconsistent with 1D models of the translational energy release based on thermal motion parallel to the surface and a repulsive energy release directed along the surface normal. The dynamics can be described by a direct, repulsive model with a transition state at extended N-2 separation, similar to the models developed for H-2 dissociation on the sane surface. We discuss the application of detailed balance to determine N-2 sticking functions S(E,upsilon,J) and, using a simple model for these functions, estimate a rotational efficacy of similar to 0.23 for sticking of N-2(upsilon = 0, J less than or equal to 24) and a vibrational efficacy of 0.7 for N-2(upsilon = 1) The dynamics are compared to the models developed for H-2 dissociation and the role of molecular chemisorption states and the local desorption site discussed. (C) 1998 American Institute of Physics.